1. Field of the Invention
This invention relates to the field of ceramics, and particularly to the field of low thermal expansion Si.sub.3 N.sub.4 composites.
2. Description of the Prior Art
Ceramic compositions produced from silicon nitride and silicon carbide are the two most advanced ceramic materials presently employed. Silicon nitride has an advantage over silicon carbide in that the former has a lower thermal expansion coefficient than the latter.
Si.sub.3 N.sub.4 can be made tougher by adding a second phase. Thus, for example zirconium oxide, ZrO.sub.2, has been added to Si.sub.3 N.sub.4, but since Si.sub.3 N.sub.4 has a low thermal expansion coefficient while ZrO.sub.2 has a relatively high thermal expansion coefficient, the thermal expansion coefficient of the resulting Si.sub.3 N.sub.4 /ZrO.sub.2 composites is increased over that of Si.sub.3 N.sub.4 alone.
Si.sub.3 N.sub.4 has also been toughened by incorporating fiber. However, fiber-reinforced Si.sub.3 N.sub.4, for example, suffers from (1) the difficulty in orienting and dispersing the fibers or whiskers, and (2) reactions between the fibers and Si.sub.3 N.sub.4 matrix. Dispersion toughened Si.sub.3 N.sub.4 has not shown significant improvements because the dispersoids (a) were too large, (b) were poorly dispersed, or (c) had a higher coefficient of thermal expansion (COTE) than the Si.sub.3 N.sub.4 matrix. A higher COTE may actually hinder toughening mechanisms. Cracks prefer to propagate normal to tensile stresses and parallel to compressive stresses. Thus, cracks are deflected around particles in hydrostatic tension (COTE dispersoid &gt;COTE Si.sub.3 N.sub.4) but attracted into particles under hydrostatic compression (COTE dispersoid &lt;COTE Si.sub.3 N.sub.4). When cracks propagate around particles, the particles accomplish little in toughening the matrix. When cracks propagate into particles, on the other hand, the particles can accomplish a high degree of toughening by dissipating energy and blunting the crack.
Accordingly, past attempts to produce toughened Si.sub.3 N.sub.4 composites have met with only limited success.
U.S. Pat. No. 3,657,063 to S. D. Brown et al, and assigned to the same assignee as the present application, discloses a high thermal shock resistant composite formed of a first layer of a composition of low expansion particulate oxide ultimately bonded to a second layer of a low expansion preformed silica material. The particulate oxide layer consists essentially of hafnia, zirconia and titania.